ASP.NET PAGE LIFECYCLE ( taken from http://www.15seconds.com/issue/020102.htm )
STAGE EVENTS/METHODS
Page Initialization Page_Init
View State Loading LoadViewState
Postback data processing LoadPostData
Page Loading Page_Load
PostBack Change Notification RaisePostDataChangedEvent
PostBack Event Handling RaisePostBackEvent
Page Pre Rendering Phase Page_PreRender
View State Saving SaveViewState
Page Rendering Page_Render
Page Unloading Page_UnLoad
When a page request is sent to the Web server, whether through a submission or location change, the page is run through a series of
events during its creation and disposal. When we try to build ASP.NET pages and this execution cycle is not taken into account, we can
cause a lot of headaches for ourselves. However, when used and manipulated correctly, a page's execution cycle can be an effective and
powerful tool. Many developers are realizing that understanding what happens and when it happens is crucial to effectively writing ASP.NET
pages or user controls. So let's examine in detail the ten events of an ASP.NET page, from creation to disposal. We will also see how to
tap into these events to implant our own custom code.
I'll set the stage with a simple submission form written in ASP.NET with C#. The page is loaded for the first time and has several server-side
Web controls on it. When the Web server receives a request for the page, it will process our Web controls and we will eventually get rendered
HTML. The first step in processing our page is object initialization.
1. Object Initialization
A page's controls (and the page itself) are first initialized in their raw form. By declaring your objects within the constructor of your C#
code-behind file (see Figure 1), the page knows what types of objects and how many to create. Once you have declared your objects within your
constructor, you may then access them from any sub class, method, event, or property. However, if any of your objects are controls specified
within your ASPX file, at this point the controls have no attributes or properties. It is dangerous to access them through code, as there is
no guarantee of what order the control instances will be created (if they are created at all). The initialization event can be overridden using
the OnInit method
2. Load Viewstate Data
After the Init event, controls can be referenced using their IDs only (no DOM is established yet for relative references). At LoadViewState event,
the initialized controls receive their first properties: viewstate information that was persisted back to the server on the last submission. The
page viewstate is managed by ASP.NET and is used to persist information over a page roundtrip to the server. Viewstate information is saved as a
string of name/value pairs and contains information such as control text or value. The viewstate is held in the value property of a hidden <input>
control that is passed from page request to page request. As you can see, this is a giant leap forward from the old ASP 3.0 techniques of
maintaining state. This event can be overridden using the LoadViewState method and is commonly used to customize the data received by the control
at the time it is populated. Figure 2 shows an example of overriding and setting viewstate at the LoadViewState event.
3. LoadPostData Processes Postback Data
During this phase of the page creation, form data that was posted to the server (termed postback data in ASP.NET) is processed against each
control that requires it. When a page submits a form, the framework will implement the IPostBackDataHandler interface on each control that
submitted data. The page then fires the LoadPostData event and parses through the page to find each control that implements this interface
and updates the control state with the correct postback data. ASP.NET updates the correct control by matching the control's unique ID with
the name/value pair in the NameValueCollection. This is one reason that ASP.NET requires unique IDs for each control on any given page. Extra
steps are taken by the framework to ensure each ID is unique in situations, such as several custom user controls existing on a single page.
After the LoadPostData event triggers, the RaisePostDataChanged event is free to execute (see below).
4. Object Load
Objects take true form during the Load event. All object are first arranged in the page DOM (called the Control Tree in ASP.NET) and can be
referenced easily through code or relative position (crawling the DOM). Objects are then free to retrieve the client-side properties set in
the HTML, such as width, value, or visibility. During Load, coded logic, such as arithmetic, setting control properties programmatically, and
using the StringBuilder to assemble a string for output, is also executed. This stage is where the majority of work happens. The Load event
can be overridden by calling OnLoad as shown in Figure 3.
5. Raise PostBack Change Events
As stated earlier, this occurs after all controls that implement the IPostBackDataHandler interface have been updated with the correct postback
data. During this operation, each control is flagged with a Boolean on whether its data was actually changed or remains the same since the
previous submit. ASP.NET then sweeps through the page looking for flags indicating that any object's data has been updated and fires
RaisePostDataChanged. The RaisePostDataChanged event does not fire until all controls are updated and after the Load event has occurred. This
ensures data in another control is not manually altered during the RaisePostDataChanged event before it is updated with postback data.
6. Process Client-Side PostBack Event
After the server-side events fire on data that was changed due to postback updates, the object which caused the postback is handled at the
RaisePostBackEvent event. The offending object is usually a control that posted the page back to the server due to a state change (with
autopostback enabled) or a form submit button that was clicked. There is often code that will execute in this event, as this is an ideal
location to handle event-driven logic. The RaisePostBackEvent event fires last in the series of postback events due to the accuracy of the
data that is rendered to the browser.
Controls that are changed during postback should not be updated after the executing function is called due to the consistency factor. That is,
data that is changed by an anticipated event should always be reflected in the resulting page. The RaisePostBackEvent can be trapped by catching
RaisePostBackEvent, as in Figure 4.
7. Prerender the Objects
The point at which the objects are prerendered is the last time changes to the objects can be saved or persisted to viewstate. This makes
the PreRender step a good place to make final modifications, such as changing properties of controls or changing Control Tree structure,
without having to worry about ASP.NET making changes to objects based off of database calls or viewstate updates. After the PreRender phase
those changes to objects are locked in and can no longer be saved to the page viewstate. The PreRender step can be overridden using OnPreRender
8. ViewState Saved
The viewstate is saved after all changes to the page objects have occurred. Object state data is persisted in the hidden <input> object and
this is also where object state data is prepared to be rendered to HTML. At the SaveViewState event, values can be saved to the ViewState object,
but changes to page controls are not. You can override this step by using SaveViewState, as shown in Figure 5.
9. The Render event commences the building of the page by assembling the HTML for output to the browser. During the Render event, the page
calls on the objects to render themselves into HTML. The page then collects the HTML for delivery. When the Render event is overridden, the
developer can write custom HTML to the browser that nullifies all the HTML the page has created thus far. The Render method takes an HtmlTextWriter
object as a parameter and uses that to output HTML to be streamed to the browser. Changes can still be made at this point, but they are reflected
to the client only. The Render event can be overridden, as shown in Figure 6 (below).
10. After the page's HTML is rendered, the objects are disposed of. During the Dispose event, you should destroy any objects or references
you have created in building the page. At this point, all processing has occurred and it is safe to dispose of any remaining objects, including
the Page object. You can override Dispose, as shown in Figure 6.